(1) Field of the Invention
The present invention relates to a process for preparing phenols from hydroperoxides of isopropyl aromatic compounds. More particularly, the invention relates to a process for recovering phenols having a high purity at a high efficiency.
(2) Description of the Prior Art
Processes for the preparation of phenols such as phenol and cresol comprising oxidizing an isopropyl aromatic compound such as cumene or cymene with molecular oxygen to form a hydroperoxide and acid-decomposing the hydroperoxide have been known in the art. Phenols, acetone simultaneously formed, unreacted hydrocarbons and by-products such as .alpha.-methylstyrene can be isolated by distillation of acid-decomposition products. The distillation residue is composed of polycondensates of various components, and phenols and other valuable components can be obtained by thermal decomposition of this distillation residue. For recovery of phenols from such thermal decomposition product, there is ordinarily adopted a process in which a distillable fraction of the thermal decomposition product is returned to the system for distillation of the acid-decomposition product and phenols contained in this fraction are recovered together with phenols contained in the acid-decomposition product. However, since compounds having a boiling point close to those of phenols are formed at the thermal decomposition, if the above-mentioned fraction of the thermal decomposition product is returned to the distillation system as described above, mingling of impurities cannot be avoided, resulting in reduction of the purity of recovered phenols and discoloration of the products. In order to prevent or eliminate these disadvantages, the load of the distillation column should be increased. Furthermore, another disadvantage is that the recovery ratio of phenols in the distillation column can hardly be maintained at a high level. More specifically, when phenol is prepared, for example, according to the cumene process, if the distillation residue is thermally decomposed, not only phenol but also by-products having a boiling point close to that of phenol, for example, phenylbutenes and 2-methyl-4-phenylpentenes are included in the thermal decomposition product. If this thermal decomposition product is returned to the distillation system, a large plate number and high reflux ratio are necessary for the distillation column so as to separate phenols by distillation without mingling of these by-products. Moreover, since removal of these by-products alone by distillation is very difficult, these by-products are gradually accumulated in the distillation system, resulting in degradation of the quality of the product.